1. Field of the Invention
The invention relates to passive optical networks, and in particular to protected passive optical networks.
2. Description of the Related Art
A Passive Optical Network (PON) is a high bandwidth point-to-multipoint optical fiber network. A PON typically consists of an Optical Line Terminal (OLT), which is connected to Optical Network Units (ONU) using only cables, optical splitters and other passive components (i.e. not transmitting signals using electricity). In a PON, signals are routed in such a way that all signals reach all interim transfer points of the PON.
Passive optical network technology has been considered a very promising solution for solving the last-mile problem. Logically a PON has a tree-like structure consisting of an optical line terminal, which is located e.g. in a central office (CO), and a plurality of optical network units, e.g. 64 ONUs. The PON technology eliminates the need for active equipment in the field between OLT and ONUs, which are commonly used in conventional networks. A PON can provide, for example, a capacity of 1 Gbps. A single link failure may result in an intolerable traffic loss, which indirectly leads to revenue loss. Thus, survivability becomes important especially when a PON is applied in a fiber-to-business and cellular-transport (CT) network environment.
Generally, there are two types of survivability architectures: a 1+1 architecture and a 1:1 architecture. The 1+1 architecture uses two overlaid PONs. The traffic is bridged into both a working PON and a protection PON. Upon receiving a signal in the OLT, the traffic is selected based on signal quality. With this approach, fast protection can be achieved. However, in this architecture, no extra traffic can be supported. Compared with the no protection case, it furthermore requires double bandwidth.
In the 1:1 architecture, under normal circumstances, the normal traffic is transmitted over the working PON. Once a failure occurs, the traffic is switched into the protection PON. The protection switching is slower relative to that of the 1+1 architecture. However, compared with the 1+1 architecture, it can either significantly reduce the spare capacity requirement or carry extra low priority traffic depending on the network design.
U.S. Pat. No. 6,351,582 discloses one solution for optimizing passive optical networks. The passive optical network includes a plurality of optical splitters/combiners, each having first and second through ports and at least one drops port. The through ports of the plurality of splitters/combiners are concatenated to form a linear arrangement having two end through ports.
FIG. 1 discloses an example of the basic structure of a ring-protected passive optical network arrangement described in U.S. Pat. No. 6,351,582. In this example, the PON includes two interfaces IF1 and IF2 within an OLT unit 110, wherein IF1 operates in active mode and IF2 in standby mode. The PON comprises a plurality of passive nodes 10-13 and 15-18, which are preferably splitters/combiners and furthermore a plurality of ONUs 14 and 19. In FIG. 1, splitters/combiners 10, 11, 15 and 16 are 1-by-2 or 2-by-1 splitters/combiners that couple optical power from and into the optical fiber.
One problem in prior-art solutions and also in the solution disclosed in FIG. 1, is that an optical signal traversing through ring splitters/combiners 10 and 11 experience optical power losses at two different stages (ring splitters/combiners 10 and 11).
The prior-art passive optical networks involve further problems that have to be overcome. In an access network, the cost is a major concern since the number of users in the access network is much less than that in metro or backbone networks. Furthermore, there exists a problem of how to effectively provide protection against a single link failure in a PON based access network without significantly increasing the cost per user.
A further problem is how to implement fast fault detection in a PON. Yet a further problem is how to fast reroute the affected traffic from the working OLT to the protection OLT.
A further problem is how to solve the attenuation problem caused by protection elements.